1. Field of the Invention
The present invention relates to an apparatus for manufacturing a Fresnel lens. More particularly, the present invention relates to a Fresnel lens manufacturing apparatus in which a Fresnel lens having high quality can be molded with great ease.
2. Description Related to the Prior Art
A Fresnel lens is characterized in that its thickness is small. A virtual concave or convex lens, having curved faces, is split with reference to a great number of minute sections. The curved faces as split into the plural sections are arranged on a plane surface to define the Fresnel lens, which is optically equivalent to the original concave or convex lens virtually plotted. The Fresnel lens is also understood as a combination of a plurality of minute lens elements which are different in direction of refraction. The Fresnel lens has an advantage in that its thickness and weight is smaller than an equivalent concave or convex lens, and in that it can be manufactured easily in mass production by injection molding. The Fresnel lens is generally used as a plate for condensing light projected from a light source. One of the two faces of the Fresnel lens can be provided with a deposit of reflective material, to obtain a Fresnel mirror, which is generally used in an overhead projector as a reflector placed on a stage for supporting a transparent original.
The produce such a Fresnel lens, plastic material is molded by injection molding. An assembly of molds having a Fresnel forming face is used. The Fresnel lens must be released from the mold assembly before the injection molding of the Fresnel lens can be terminated. For the purpose of releasing the Fresnel lens in better condition, it is proposed in JP-A (Japanese Patent Laid-open Publication No.) 4-69217 that plural release pins are arranged through the mold assembly, and are adjusted to project toward the Fresnel lens progressively farther in directions away from the center of the Fresnel lens.
It is normal to form Fresnel plates by injection molding, compression forming, or casting. A coating of transparent protective membrane is applied to the Fresnel plate. A Fresnel mirror is produced by depositing a reflective membrane on the Fresnel plate and by applying the protective membrane thereto.
The Fresnel lens has minute ridges and grooves in the thinly formed plate formed by injection molding. The contact pressure between the mold assembly and the Fresnel leans is remarkably high. If only ejector pins are used for releasing or separating the Fresnel lens from the molds, such pins are required to press with great force on the Fresnel lens. Such release pins press only points on the Fresnel lens, namely at very small areas. The points receive excessive pressure from the pins, to such an extent as to bend the Fresnel lens, and damage its optical performance seriously. The pins are likely to tear the Fresnel lens. If the Fresnel lens is designed to have a large area, a much greater number of release pins must be arranged correspondingly. This makes it very difficult to construct an associated mold assembly.
After the forming of the Fresnel face of the plate the protective membrane is applied, and the reflective membrane is deposited. This requires steps additional to the forming of the Fresnel face, makes the manufacture more difficult and complex, and raises the cost of manufacture. In the Fresnel plate of a reflective type, in particular, it is very difficult to coat the deposited reflective membrane with the protective membrane at a regular thickness. This is a problem in degrading optical performance of condensing light, and lowering the durability of the protective membrane.